from mailing list:

For nginx/apache you have a rewrite rule to serve the static files so how the ruby server would react if a request for a static file hit it doesn't really matter. However serving them with ruby is really a very wasteful and slow way of delivering static files (which is why it by default doesn't in production mode).

My vote goes to better documentation and leaving the default not to server static files. Otherwise you'll mask what essentially is a configuration error.